The invention is in the field of environmental control, and particularly concerns an apparatus and a method for accelerating the reaction of an environmental test system to the environmental response of a device under test in the system.
In the process control field, a system for performing an industrial process (the controlled system) gives forth its desired product in response to automatic means (the control system) which dynamically correct process parameter values of the controlled system in a manner which makes the result of the process fall within a predetermined range of values.
Control systems employ a technique called "cascading". In this regard, a control system can include more than one compensating unit which responds to the current values of a first set of process parameters by producing an operating signal. In a cascaded control system, the operating point signal of one control unit is provided as one input to another compensating unit.
Conventionally, cascade control has been applied to control relatively "slow" primary physical processes through secondary control of "faster" processes. For example, in a heat exchanger system, steam is introduced into a heat-exchanger to provide a source of thermal energy for heating a reagent. The slow process of heating the reagent is controlled in a primary unit by manipulation of the heat exchange process in response to the present heat of the reagent and the desired heat. The primary unit produces a thermal change value. The value of the available heat parameter (steam flow) is controlled in a secondary compensation unit having a fast reaction to the thermal change value and steam heat. The secondary unit produces a signal which quickly controls the flow of steam.
A well-known industrial process is embodied in an environmental chamber which is used to test a device ("device under test" or "DUT"). The industrial process to be controlled is one or more environmental effects produced by the chamber. For example, the thermal response of a device is evaluated in an environmental chamber having means for establishing the temperature of its interior and for changing that temperature at a selected rate to another temperature. The temperature of the DUT changes in response to the change in environmental chamber temperature at a rate determined by physical characteristics of the device. The temperature responses of the device and the environmental chamber are both relatively slow industrial processes. Furthermore, the thermal characteristics of the device will vary under varying test conditions of pressure, humidity, and temperature. Last, control of the chamber temperature as a secondary variable in a cascade control system can lead to high, and possibly fatal, thermal stress on the device under test.